1. Field of the Invention
The present invention relates to a wireless power supply technique.
2. Description of the Related Art
In recent years, in order to supply electric power to an electronic device, contactless power transmission (which is also referred to as “contactless power supply” or “wireless power supply”) has begun to come into commonplace use. In order to advance the compatibility of products between manufacturers, the WPC (Wireless Power Consortium) has been organized, and the WPC has developed the Qi standard as an international standard.
FIG. 1 is a diagram showing a configuration of a wireless power supply system 100 that conforms to the Qi standard. The power supply system 100 includes a power transmission apparatus 200 (TX) and a power receiving apparatus 300 (RX). The power receiving apparatus 300 is mounted on an electronic device, examples of which include cellular phone terminals, smartphones, audio players, game machines, and tablet terminals.
The power transmission apparatus 200 includes a transmission coil 202 (primary coil), a driver 204, a controller 206, and a demodulator 208. The driver 204 includes an H-bridge circuit (full-bridge) circuit or otherwise a half-bridge circuit. The driver 204 applies a driving signal S1, configured as a driving current or otherwise a driving voltage, for example, to the transmission coil 202 such that an electric power signal S2 is generated at the transmission coil 202 in the form of an electromagnetic signal. The controller 206 integrally controls the overall operation of the power transmission apparatus 200. Specifically, the controller 206 controls the switching frequency of the driver 204 or otherwise the duty ratio of the switching of the driver 204 so as to adjust the electric power to be transmitted.
In the Qi standard, a protocol is defined for communication between the power transmission apparatus 200 and the power receiving apparatus 300, which enables information transmission from the power receiving apparatus 300 to the power transmission apparatus 200 via a control signal S3. The control signal S3 is transmitted from a reception coil 302 (secondary coil) to the transmission coil 202 in the form of an AM (Amplitude Modulation) modulated signal using backscatter modulation. The control signal S3 includes electric power control data (which will also be referred to as a “packet”) which indicates an amount of electric power to be supplied to the power receiving apparatus 300, and data which indicates the particular information for identifying the power receiving apparatus 300. The demodulator 208 demodulates the control signal S3 included in the current or otherwise the voltage applied to the transmission coil 202. The controller 206 controls the driver 204 based on the power control data included in the control signal S3 thus demodulated.
The power receiving apparatus 300 includes the reception coil 302, a rectifier circuit 304, a capacitor 306, a modulator 308, a load circuit 310, a controller 312, and a power supply circuit 314. The reception coil 302 receives the electric power signal S2 from the transmission coil 202, and transmits the control signal S3 to the transmission coil 202. The rectifier circuit 304 and the capacitor 306 rectify and smooth a current S4 induced at the reception coil 302 according to the electric power signal S2, thereby converting the current S4 into a DC voltage.
Using electric power supplied from the power transmission apparatus 200, the power supply circuit 314 charges an unshown secondary battery or steps up or otherwise steps down the DC voltage Vdc, so as to supply the DC voltage to the controller 312 and other load circuits 310.
The controller 312 monitors the amount of electric power supplied to the power receiving apparatus 300, and accordingly generates electric power control data which indicates the amount of power transmission. The modulator 308 modulates the control signal S3 including the electric power control data so as to modulate the coil current that flows through the reception coil 302, thereby modulating the coil current and coil voltage applied to the transmission coil 202.
A state can occur in which there is an electro-conductive foreign object such as a piece of metal or the like between, or otherwise in the vicinity of, the transmission coil 202 and the reception coil 302. In a case in which wireless power supply is performed in this state, a current flows through the foreign object, leading to power loss. Also, such a case leads to a problem of the foreign object heating up. In view of such a situation, foreign object detection (FOD) has been designed according to the WPC1.1 (System Description Wireless Power Transfer Volume I: Low Power Part 1: Interface Definition Version 1.1) specification.
In such FOD, the electric power transmitted by the power transmission apparatus 200 is compared with the electric power received by the power receiving apparatus 300. When the difference between them is greater than an allowable value, judgement is made that a foreign object exists.